Both eyes of a healthy individual are aligned and the visual axes are parallel under most viewing conditions. Deviation from this state may cause diplopia. Strabismus is a visual disorder in which the eyes are misaligned. Strabismus, (i.e. double vision), which is present in about 4% of children in the U.S., prevents stereopsis (i.e. depth perception) and can lead to development of amblyopia (i.e. lazy eye). Amblyopia is a decrease in vision in one or both eyes that cannot be accounted for by any structural abnormality and cannot be treated by optical devices. Amblyopia can be engendered by childhood strabismus, as a result of the child's brain ability to disregard the image from the deviating eye, in order to avoid double vision. If this condition is not treated in early childhood the amblyopia might become permanent.
Eye misalignments are classified to subtypes with different clinical implications. A major factor is whether the eyes deviate constantly (i.e. Heterotropia) or only under certain circumstances (i.e. heterophoria). The latter may lead to eye fatigue, reading disorders and decrements in stereopsis, but not for amblyopia. Untreated heterotropia leads to amblyopia in most cases when one eye is constantly deviating. In strabismus, there may be a cosmetic blemish if the angle between the visual axes is large enough.
As with other binocular vision disorders, the primary therapeutic goal for strabismus is a comfortable, single, clear, normal binocular vision at all distances and directions of gaze. Strabismus is usually treated with a combination of eyeglasses and surgery.
The earlier the treatment of strabismus is initiated in infancy, it is less likely to develop amblyopia. Starting treatment at as young an age as possible may ensure the development of the best possible visual acuity in both eyes and enable stereopsis. Strabismus is generally treated by preventing good vision in the non-deviating (non strabismic) eye, by physical, pharmacology or optical blurring. The treatment of amblyopia does not change the angle of strabismus which, if large enough, is treated surgically.
A patient with a constant deviation eye turn of significant magnitude, is very easy to notice. However, a small magnitude or intermittent strabismus can easily be missed upon casual observation.
Adults might also develop strabismus; they usually do not develop amblyopia, but a double vision. Except for the discomfort of double vision, people of all ages may experience psychosocial difficulties if they have noticeable strabismus. Successful surgical correction of strabismus has positive effects on psychological well-being, even when implemented with adult patients. Although not a cure for strabismus, prism lenses can also be used to provide comfort for patients and to prevent double vision from occurring. The prisms can usually enable single vision in only single gaze position since the angle between eyes might change according to the visual gaze direction.
Thus early detection of strabismus in young children is of paramount importance in avoiding amblyopia and increasing the chances of developing proper binocular vision. Determining the origin, the type and the angles of strabismus is important for treatment decisions such as which of the external eye muscles to operate, and to what degree.
There exist several strabismus and ocular motility examinations as follows:
Pupillary light reflex examination, in which a patient is directed to look at a point of light held a meter away. If the light reflections are located symmetrically in each pupil, the eyes are parallel. Asymmetric reflections indicate possible strabismus. The angle of the deviation may be measured by bringing the reflection to symmetry by a prism of appropriate strength. However, this test is inaccurate, but is the only one possible in most babies.
A cover test is performed while the patient looks at a near or distant target, and one eye is covered, while the uncovered one observes. The patient is required to fixate on a target while the examiner repeats the test several times using prism lenses with a variety of strengths and directions (up, down, in and out) in order to measure the misalignment parameters. This procedure might take a long period of time, up to 30 minutes when testing infants or toddlers. Asking young children to cooperate and fixate for such a long time is challenging and requires the specialist to be very creative.
A prism cover test, in which the degree of eye misalignment is established by neutralizing the uncovered eye movements using a prism bar or prism lenses. This test should be performed separately for horizontal and vertical strabismus. However, this test requires a patient's cooperation and is quite complicated to perform.
The cover and prism cover tests require the subjective judgment of the specialist to determine the existence of eye movements between consecutive cover/uncover steps.
There are also further complicated tests such as tests using a Hess screen which are also subjective, more complicated, and which are time consuming.
All the above tests are complicated to perform and are imprecise, prolonged, and have to be performed by a specialist. Furthermore, they are difficult, and in many cases impossible to perform on infants and toddlers.